Locally advanced bladder cancer develops into lethal metastatic disease, commonly to the lung, in half of all patients. Using both clinical and laboratory methodologies, we identified a new molecular pathway that regulates metastatic lung colonization (growth of micrometastases into clinical disease), components of which are altered in human disease. This pathway involves RhoGDI2 (ARHGDIB, Ly-GDI), c-Src, Rac1, endothelin-1 and versican. Microarray profiling of lineage related human bladder cancer cell lines of differing lung metastatic ability coupled with analysis of human bladder cancers at different stages identified RhoGDI2 as a candidate inhibitor of metastatic colonization. We subsequently found decreased protein expression of RhoGDI2 is an independent predictor of metastasis development in patients with bladder cancer. We also found that c-Src regulates this pathway in part by phosphorylating RhoGDI2, which increases its potency as a suppressor of lung colonization. Consistent with these data, c-Src expression decreases as a function of bladder tumor stage, and this decrease appears mutually exclusive with reduced RhoGDI2 expression. RhoGDI2, like other RhoGDIs, binds and inhibits Rho family GTPases. Investigation of these proteins yielded the surprising result that inhibition of metastases by RhoGDI2 correlated with activation of Rac1. In concurrent work, aimed at finding effectors of this pathway, gene array studies led us to identify endothelin-1 and versican as mRNAs downregulated by RhoGDI2 and upregulated in high stage human bladder cancer. Both are known macrophage chemoattractants and depletion of either protein reduced macrophage migration toward metastatic bladder cancer cells. RhoGDI2 re-expression in bladder cancer cells was also associated with decreased macrophage infiltration of lung metastases in mice. Since tumor associated macrophages (TAMs) have been found to promote metastatic colonization in many types of cancer, recruitment of macrophages by endothelin-1 and versican may mediate enhancement of metastatic growth in cells with decreased RhoGDI2. Hence, our Guiding Hypothesis is that active RhoGDI2 inhibits metastatic colonization by downregulating endothelin-1 and versican expression, which reduces tumor associated macrophage recruitment to the metastatic site. Specific Aims are proposed to test this hypothesis, develop novel prognostic tools for advanced bladder cancer and identify new targets for therapy in patients. Aim 1: Evaluate the role of RhoGDI2 as a regulator and predictor of metastasis. Aim 2: Elucidate the mechanism by which RhoGDI2 activates Rac1 to inhibit metastasis; Aim 3: Evaluate effectors of metastasis suppression by RhoGDI2. Completion of these aims will contribute clinically useful knowledge as well as providing mechanistic insights into a novel pathway that suppresses bladder cancer metastasis. Doing so will also provide a new paradigm of how metastasis suppressor proteins can function by affecting innate immunity and facilitate translational approaches aimed at development of rational therapies for patients with bladder cancer.